FIG. 1 illustrates an imaging system 1 described in U.S. Pat. No. 4,983,911 for observing different voltages present in a substrate 2 under test. The substrate 2 may be, for example, a semiconductor wafer, or chip, or a liquid crystal display (LCD) panel. If the electrical continuity and discontinuity of the various conductors on the substrate are to be tested by the system, some or all of the conductors on the substrate are supplied with a voltage from a voltage supply (not shown) so that electric fields from the energized conductors are present at certain sites on the surface of the panel.
In order to observe the electric fields so as to detect conductor continuities and discontinuities, light from a source of optical energy 3, such as a xenon, sodium, or quartz halogen lamp, a pulsed or continuous laser or the like, is channeled into a source beam 4. This source beam 4 may be expanded and collimated with a beam expander 5 to produce a collinear input beam 6A. This input beam 6A passes from the expander 5, through a beam splitter 7, through an electrically conductive yet optically transparent layer 8 disposed on a top surface of an electro-optic modulator block 9, through the bulk of the electro-optic modulator block 9, and to a reflective nonconductive layer 10 disposed on a bottom surface of the electro-optic modulator block 9. This input beam 6A then reflects off the reflective nonconductive layer 10, passes back through the bulk of the electro-optic modulator block 9 as an output beam 6B, passes back through the electrically conductive yet transparent layer 8, is separated from the collinear input beam 6A by beam splitter 7, and is directed into a focussing lens 11 and to an area light sensor 12 by beam splitter 7. Light sensor 12 may, for example, be a charge coupled device (CCD) camera.
The electro-optic modulator block 9 modulates light passing through the electro-optic modulator block 9 in response to the electric field present across the electro-optic modulator block 9. For example, one type of electro-optic material changes the polarization of light in response to an electric field across it.
When the bottom surface of the electro-optic modulator block 9 is placed over a substrate 2 having different areas of its surface charged with different voltages, the different voltages in the substrate cause different portions of the electro-optic material to experience different electric fields. By using light sensor 12 to observe the different characteristics of the light passing through the electro-optic modulator block 9, continuities and discontinuities of the conductors in the underlying substrate are detected.
The conductive yet optically transparent layer 8 disposed on the top surface of the electro-optic modulator block 9 provides an equipotential plane to enable a voltage differential to appear across electro-optic block 9 corresponding to the varying voltages on the substrate. Accordingly, when the various conductors in the substrate are charged with different voltages in relation to this equipotential plane 8, various portions of the electro-optic modulator block will experience different electric fields. Additional details of the imaging system of FIG. 1 are explained in U.S. patent application Ser. No. 07/481,429, filed Feb. 5, 1990, entitled "Voltage Imaging System Using Electro-Optics", now U.S. Pat. No. 4,983,911.